In recent years, according to the progress of diagnostic imaging apparatuses including magnetic resonance imaging (MRI), etc., it has become easy to perform diseased portion diagnosis of the spinal cord and peripheral nerves due to oppression lesions. However, there are many cases, for example, in which no symptom is found even when there is apparently an oppression according to an image. Therefore, it is impossible to truly diagnose a functionally diseased portion of the spinal cord or the peripheral nerves by using only morphological information based on images, and thus, nerve function diagnostics using electrophysiological techniques are still indispensable tests.
In order to perform detailed diseased portion diagnosis, it is the best way to measure a nerve evoked potential by using the inching technique. However, because an electrical current receives strong influence from surrounding tissue in nerves deep inside away from the body surface, especially in the spinal cord, it is difficult to accurately evaluate nerve functions from the body surface. Therefore, the spinal cord evoked potential is measured by intraoperatively setting electrodes near the spinal cord, or by preoperatively and percutaneously inserting catherter electrodes in the extradural space or the subarachnoid space. Inserting the catherter electrodes is invasive and requires skill, and cannot be considered as a test which can be easily performed for diagnosis. Therefore, a non-invasive and easy electrophysiological technique is desired.
It should be noted that, when currents flow, magnetic fields are generated around the currents in accordance with the right-handed screw rule. The magnetic field has a property of receiving almost no influence from the biological tissue such as bones and soft tissue, and it is known that theoretically the biomagnetic field measurement has higher spatial accuracy compared to the potential measurement. The biomagnetic field measurement is a technique for measuring, from outside of a living body, microscopic magnetic fields generated according to activities of nerves and muscles of the living body, and analyzing the behavior of the activity sources. A biomagnetic field measurement system has been developed and introduced to medical sites in which system a multi-channel magnetic measurement apparatus utilizing superconducting quantum interference devices (SQUID) is used.
Currently, the biomagnetic field measurement has been especially applied to the field of brain research, and thus, brain activities have been identified with high spatial accuracy. Further, medical sites of mainly spine/spinal cord surgery and peripheral nerve surgery have been paying attention to the biomagnetic field measurement system as an effective technique for diagnosing a nerve signal propagation disorder in the case of occurrence of a nervous system disorder by measuring magnetic fields according to activities of the nervous system other than the brain such as the spinal cord and peripheral nerves. It should be noted that several papers are known in which experimental examples of measuring the spinal cord evoked magnetic fields are described.
In order to measure nerve magnetic fields of a living body, a nerve stimulation apparatus is needed together with the magnetic measurement apparatus. The peripheral nerves are stimulated by stimulation currents according to the nerve stimulation apparatus, and magnetic fields generated by nerve activities due to the stimulation are measured by the magnetic field measurement apparatus. By synchronizing the magnetic field measurement with the current stimulation, the measured magnetic fields can be identified as generated by the currents flowing in the peripheral nerves and the spinal cord. However, it is difficult to apply stable stimulation currents to the peripheral nerves. For example, a subtle change of positional relationship between nerves and stimulation electrodes disables an appropriate peripheral nerve stimulation, and thus, it becomes difficult to apply optimal peripheral nerve stimulation, which is a problem.
Therefore, a technique has been studied in which multiple stimulation negative electrodes and a circuit for selecting the best electrode of the electrodes are used. In the technique, an electrode should be selected which stimulates the nerve appropriately, and the nerves should be stimulated with high efficiency (e.g., refer to PTL 1). With the above technique, it is possible to realize a nerve stimulation apparatus which percutaneously applies optimal stimulation to the nerves.